Fabrication of polymeric nanofibers may be used in a wide variety of applications such as in the fields of sensors and actuators, energy storage, smart textiles, optoelectronics, tissue engineering, medical device fabrication, prosthetics, drug delivery, microresonators, and piezoelectric energy generators. Several processes have been developed to tailor the properties of polymeric nanofibers to suit the particular needs of each application. These polymeric nanofiber modification techniques include chemical modification, surface deposition of metals, functional doping, and composite formation. Polymeric nanofibers can also be pyrolyzed to yield thinner carbon nanofibers, opening up an even wider range of applications, including electrochemical sensors and energy storage.
Polymeric nanofibers may be useful as diodes. The Schottky diode is a semiconductor diode with a low forward voltage drop and a fast switching action. When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6-1.7 volts, while a Schottky diode voltage drop is between approximately 0.15-0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.
To form a Schottky diode, a metal-semiconductor junction is formed between a metal and a semiconductor, creating a Schottky barrier instead of a semiconductor-semiconductor junction as in conventional diodes. Typical metals used are molybdenum, platinum, chromium or tungsten; and the semiconductor would typically be N-type silicon. The metal side acts as the anode and N-type semiconductor acts as the cathode of the diode. This Schottky barrier results in both fast switching and low forward voltage drop.
One of the key factors in the utilization of polymeric nanofibers in many of the aforementioned applications is the ability to accurately control the physical properties and positioning (patterning) of the produced nanofibers. One option for continuous patterning of polymer nanofibers is far-field electrospinning (FFES), which is a well-known technique to produce polymeric nanofiber mats in large quantities. Conventional Far-Field Electrospinning (FFES) involves application of 10 to 15 kV to propel a polymer jet from a biased syringe nozzle towards a grounded substrate electrode. Typically in FFES, the syringe-to-substrate distance is in the range of several centimeters, e.g., around 10-15 cm. Unfortunately, the high voltage used in FFES causes bending instabilities in the jet that leads to chaotic whipping motion of the depositing nanofibers. This whipping motion makes it difficult to control the position of where the nanofibers land on the substrate.
Although work has been carried out to achieve alignment of nanofibers along a prescribed direction through the use of a rotating drum collector, and by using electrical field manipulation, precise 2D and 3D patterning is still very difficult to achieve with FFES.
Recent efforts on a variant of electrospinning called near-field electrospinnning (NFES) produced some encouraging initial results, opening up a possibility of achieving scalable precision patterning with polymeric nanofibers. NFES offers the advantage of large scale manufacturability (inherent in electrospinning) combined with controlled electric field guidance (due to a reduced distance between the source and collector electrodes). However, the reported efforts required the use of electric fields well in excess of 200 kV/m for continuous NFES operation so that the resulting polymer jets still exhibit bending instabilities and thus limited control of polymeric nanofiber patterning. For example, Chang et al. disclose continuous near-field electrospinning for large area deposition of orderly nanofiber patterns using an electric field of at least 1,200 kV/m (applied voltage of 600V to syringe needle). See Chang et al., Continuous Near-Field Electrospinning For Large Area Deposition of Orderly Nanofiber Patters, Appl. Phys. Lett. 93, 123111 (2008).